1. Field of the Invention
The present invention relates to a position indicator suitable to be used to detect a pen pressure, a variable capacitor adapted to detect the pen pressure, and an input device having the position indicator.
2. Description of the Related Art
In recent years, a coordinate input device has been used as an input device for a personal computer for example. Such a coordinate input device is configured by a pen-like position indicator and a position detecting device having an input surface where input operation such as a pointing operation, a handwritten character input operation and a hand-drawn illustration input operation is performed by using the position indicator.
Further, a variable capacitor such as disclosed in Japanese Patent No. 3150685 (referred to as “Patent Document 1” hereinafter) is used to detect a pen pressure of the position indicator. Further, Japanese Unexamined Patent Application Publication No. 2001-319831 (referred to as “Patent Document 2” hereinafter) discloses a technique in which an electrode on one surface of a variable capacitor is divided into two so that signals become easy to be taken out.
FIGS. 16A and 16B show a concrete configuration of a variable capacitor 200 disclosed in Patent Document 1. The variable capacitor 200 includes a substantially disc-shaped dielectric 201, a first electrode 202 attached on one surface 201a of the dielectric 201, and a second electrode 203 provided on the side of the other surface 201b of the dielectric 201. The second electrode 203 is a flexible member, and is arranged on the side of the other surface 201b of the dielectric 201 via a ring-shaped spacer 204. Further, a rod 210 is provided on the side of the second electrode 203 opposite to the dielectric 201 via an elastic body 205.
A first terminal 206 is provided on the side of one surface of the first electrode 202. The first terminal 206 includes a disc-shaped flange portion 206a, and a lead portion 206b extending from substantially the center of one surface of the flange portion 206a. When a pen pressure is applied to the rod 210, the flange portion 206a is brought into contact with the one surface of the first electrode 202, so that the flange portion 206a is electrically connected to the first electrode 202.
A second terminal 207 is provided on the end portion of the second electrode 203. Similar to the first terminal 206, the second terminal 207 includes a disc-shaped flange portion 207a, and a lead portion 207b extending from substantially the center of one surface of the flange portion 207a. When a pen pressure is applied to the rod 210, the flange portion 207a is brought into contact with the end portion of one surface of the second electrode 203, so that the flange portion 207a is electrically connected to the second electrode 203.
In the variable capacitor 200 having the aforesaid configuration, in a state where there is no pressure or displacement applied to the rod 210 (namely, in a state where the variable capacitor is in the initial state), the other surface 201b of the dielectric 201 and the second electrode 203 are spaced apart from each other by the spacer 204 (see FIG. 16A). Thus, an air layer 208 is formed between the other surface 201b of the dielectric 201 and the second electrode 203, the air layer 208 having the same thickness as that of the spacer 204. The capacitance value between the terminal 206 and the terminal 207 in the initial state (i.e., the initial capacitance) is substantially a serially combined capacitance of the capacitance contributed by the dielectric 201 and the capacitance contributed by the air layer 208, which has a relative dielectric constant of 1.0, and therefore the capacitance value is very small.
However, if a pressure is applied to the rod 210, the second electrode 203 will be bent toward the side of the dielectric 201, and therefore the thickness of the air layer 208 will become smaller than that of the spacer 204. At this time, since the capacitance contributed by the air layer 208 increases in inverse proportion to the thickness of the air layer 208, the capacitance between the terminal 206 and the terminal 207 will increase due to the increase of the capacitance contributed by the air layer 208.
Then, after the second electrode 203 is brought into contact with the other surface 201b of the dielectric 201, the capacitance value between the terminal 206 and the terminal 207 increases substantially in proportion to the contact area between the second electrode 203 and the other surface 201b of the dielectric 201. The pressure (the pen pressure) applied to the rod 210 is detected by detecting the change of the capacitance value between the first terminal 206 and the second terminal 207.